help: fix leaking return value from `help_unknown_cmd()`

help: fix leaking return value from `help_unknown_cmd()`

While `help_unknown_cmd()` would usually die on an unknown command, it
instead returns an autocorrected command when "help.autocorrect" is set.
But while the function is declared to return a string constant, it
actually returns an allocated string in that case. Callers thus aren't
aware that they have to free the string, leading to a memory leak.

Fix the function return type to be non-constant and free the returned
value at its only callsite.

Note that we cannot simply take ownership of `main_cmds.names[0]->name`
and then eventually free it. This is because the `struct cmdname` is
using a flex array to allocate the name, so the name pointer points into
the middle of the structure and thus cannot be freed.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>